The end is in sight, but for many, there is still time for soybean aphids to impact yields in Minnesota. The questions of whether to spray, when to spray, and what to spray on these late-season aphids was recently addressed by Robert Koch, Bruce Potter and Ken Ostlie in this Minnesota Crop News piece. With widespread rains, low temperatures and a continuously developing soybean crop, it is a critical time to evaluate (or re-evaluate) plans for spraying these late-season aphids.
Treating late-season aphid populations requires a much more complex decision-making process than does an application made against early-season infestations. The decision whether to, and how to treat aphids is now based on these primary factors:
1) Crop growth stage
2) Current aphid numbers
3) Recent aphid numbers
4) Previous applications
5) Pre-harvest interval
Crop growth stage – Although we do not have as much data regarding return on investment of late-season applications for control of soybean aphid, studies do support ceasing control measures starting at the R6.5 growth stage. The R6 stage can be easily be identified by examining pods four nodes down from the top of the plant. If any of the pods contain soybeans that fully fill the width of the pod cavity, the soybeans are at the R6 stage. The R6.5 stage occurs about seven to 10 days after R6 and often coincides with the early stages of leaf drop in the lower canopy. At this stage, the soybean aphids are likely to soon begin emigrating out of soybean fields.
Many soybean fields in Minnesota are likely to be at the R6 stage by this time; however, late-planted or very long season soybeans will continue to be in the R5 stage for some time. Pay particular attention to these fields as they are the most likely to respond to a treatment if aphid populations support it.
Current aphid numbers – One should continue to use a treatment threshold of 250 aphids per plant and 80 percent or more of the plants infested with aphids through the R5 growth stage. Aphids can cause yield loss in the early R6 stage, but higher populations of aphids can be supported without economic loss during this stage.
Recent aphid numbers — The cumulative affect of aphids is a primary driver of yield loss. Therefore, if aphid numbers have been very low due to an earlier application of insecticide or if the field has been colonized very late in the R5 stage and has relatively low numbers, the chances of a positive return on investment for spraying are reduced.
Previous applications – If a field surpasses the economic threshold for aphids after an earlier application of insecticide, please apply a product from a different insecticide “group” (mode of action) than the earlier application. Insecticide groups are indicated on the product labels. This will help ensure effective control and will reduce the pressure on the aphid population to develop resistance to insecticides.
Pre-harvest interval – As always, follow the product label. An additional factor to review with late-season applications is the pre-harvest interval (PHI). Do not apply products with long PHI’s.
Aphid management has been complicated this week by several days of widespread rains and windy conditions. If aerial applications have been planned but delayed, continue to scout fields regularly to ensure that the aphids remain a threat and that your soybean crop has not moved into the R6.5 growth stage.
Seth L. Naeve is a University of Minnesota Extension Soybean Agronomist and Associate Professor of Agronomy and Plant Genetics. Reach Seth at 612-625-4298 or email him at firstname.lastname@example.org.